1. Field of the Invention
This invention relates to an ionizer, and more particularly, to an ionizer for eliminating static electricity on a large size substrate.
2. Description of Related Art
Static electricity is usually caused when certain materials are rubbed against each other. An object that has static electricity charges built up on its surface has an electrical force field coming from the surface. This field will attract neutral particles and particles with opposite charges so that the surface of the object is easily contaminated. As a result, in the fabrication processes demanding serious cleanliness, such as lithographic and etching steps of semiconductor fabrication process, the formation of static electricity should be prevented.
The methods used to solve the problem of static electricity accumulation are: (1) grounding the apparatus to guide the charges from the surface of the object to the environment; (2) adjusting the humidity to prevent the formation of static electricity charges; (3) adding metal shielding to prevent the object from the influence of outer power source. Other to the methods mentioned above, an ionizer is usually used to spray charges on the surface of the object to neutralize static electricity charges.
According to the principle of static electricity elimination, the ionizer can be sorted into active, passive, air-added, and non air-added, wherein the active ionizer can be further sorted into DC type and AC type according to the connected power.
Referring to FIG. 1, which shows a traditional air-added AC ionizer. The ionizer comprises a bar 10 and a plurality of pin sets 20. Pressured air passes through the bar 10, and an AC power line 11 is located in the bar 10. The pin sets 20 are located on the bar 10 at intervals of 5–10 cm.
Referring to FIG. 2, which shows a pin set 20 of the traditional ionizer shown in FIG. 1. Each pin set 20 comprises a pin 21 electrically connecting to the power line 11 and a nozzle 22 located around the pin 21. The pin 21 is used to discharge charges and the nozzle 22 is used to spray the charges on the substrate. The spraying angle of the nozzle 22 shown in this figure is about 35 degrees, which is broadly used in industry.
When eliminating static electricity, the power line 11 is connected to an AC power supply and point discharge happens at the pins 21. The discharged charges are electrically alternated according to the AC power. By using the nozzles 22 to spray the pressured air on the substrate, the charges created near the pins 21 are moving to the substrate to neutralize the charges thereon.
However, in the condition of large size substrate, several weaknesses of the traditional ionizer are concerned.
Referring to FIG. 3, which shows static electricity eliminating in process by using the traditional ionizer. The covering angle of the ionizer is about 35 degrees, the distance between the ionizer and the substrate is D, and the related eliminating area is F. The length of the substrate is L, and the width is W. The substrate moves respect to the ionizer at a moving speed of V. When the size of substrate increases, the covering area maybe too small to offer proper static electricity eliminating efficiency.
Referring to FIG. 4, which shows static electricity eliminating in process by using the traditional ionizer when the length of the substrate is doubled. If the moving speed is fixed, the time needed to finish the static electricity eliminating process will elongate from L/V to 2L/V. On the other hand, if the time needed to finish the static electricity eliminating process is constrained to L/V, the speed should increase to 2V and the charges density received by the substrate is half the condition shown in FIG. 3. Therefore, the static electricity charges on the substrate surface may not be fully neutralized.
Referring to FIG. 5, which shows the static electricity eliminating in process by using a traditional ionizer when the distance between the ionizer and the substrate is doubled. If the covering angle of the ionizer fixes, the increasing of the distance D between the ionizer and the substrate will result in a wider eliminating area F′ (F′>F). However, at the same time, the charge density decreases in a ratio square to the increasing ratio of the distance D, and the chance of opposite charges neutralization increases as the distance D increasing. Consequently, the density of the charges received by the substrate is too low to achieve reasonable neutralization efficiency.
Referring to FIG. 5, the distance between the ionizer and the substrate increases from D to 2D, so that the space for setting the ionizer is doubled and the cost is increased.